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(11) | EP 0 571 632 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 158(3) EPC |
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| (54) | POLYCRISTALLINE SILICON THIN FILM AND PROCESS FOR FORMING IT AT LOW TEMPERATURE |
| (57) A polycrystalline silicon thin film, which can be formed at a temperature below 400
°C on a cheap glass base plate (3) such as a blue plate glass, or on a glass base
plate (3) provided with metallic electrodes or transparent electrodes, and whose hydrogen
content is less than 5 atom %. The polycrystalline silicon thin film can be formed
at such a temperature by repeating several times the steps of forming an amorphous
silicon film on the glass plate by, e.g., a CVD technique and then exposing the film
to a hydrogen plasma for a predetermined time. |
Technical Field
[0001] The present invention relates to a polycrystalline silicon (polysilicon) thin film fabricated on an inexpensive glass substrate and large-area low-temperature fabrication method thereof, having potential application for thin film transistors and thin-film solar cells.
Background Art
[0002] Since conventional methods for the fabrication of a polysilicon thin film requires high temperature more than 650 °C, a glass substrate with a softening point is less than 650 °C is not able to be used. In order to avoid diffusion of impurities, it is necessary to use a highly pure quartz glass substrate among glass substrates with a softening point above 650 °C.
[0003] Recently, low temperature producing method by using excimer laser is intensively researched. In this method, however, small are a such as 5 mm squares is able to be crystallized, so that it is necessary to move a substrate in order to obtain complete crystallization over a large area. When the substrate is moved, nonuniformity of crystallization arises in the boundary region depending upon the moving speed of the substrate, so that it is difficult to uniformly form the polysilicon thin film on a large area.
[0004] The prevent invention is made to disolve the above-mentioned disadvantages of the conventional art, namely, it is an object of the prevent invention to provide a polysilicon thin film and a low temperature fabrication method thereof in which a inexpensive glass substrate such as a soda glass is used as a substrate.
Disclosure of the Invention
[0005] The polysilicon thin film of the present invention is a polysilicon thin film fabricated on a glass substrate characterized in that a hydrogen content of the film is not more than 5 atomic%.
[0006] The polysilicon thin film of the present invention is preferably fabricated over the surface of a metal electrode or transparent electrode, wherein both said metal or transparent electrodes are fabricated on the surface of a glass substrate. Furthermore, the thin polysilicon film is preferably fabricated using repetitions comprising the successive fabrication of amorphous silicon film by the CVD method or PVD method, exposure of the film to hydrogen plasma for a set period of time, and further fabrication of amorphous silicon film.
[0007] Moreover, in the present invention, it is preferable that the hydrogen plasma is generated by ECR discharge using permanent magnets, and that pressure thereof is not more than 100 mTorr.
[0008] The method of the present invention is a method fabricating a polysilicon film on a glass substrate characterized in that the polysilicon film is formed at a temperature not more than 400 °C.
[0009] In addition, the fabrication method of the present invention preferably comprises multple repetitions of a process comprising the fabrication of an amorphous silicon film by the CVD method or PVD method or PVD method and subsequent exposure of the amorphous silicon film to a hydrogen plasma for a set period of time .
[0010] Moreover, in the method of the present inventions, it is preferable that the above-mentioned CVD method or PVD method and the exposure to hydrogen plasma are carried out in the same chamber.
Brief Description of the Drawings
[0011]
Fig.1 is a schematic diagram of a film forming apparatus applied to the film fabrication method of the prevent invention;
Fig.2 is the Raman spectrum of an embodiment of the present invention; and
Fig.3 is the X-ray diffraction spectrum of an embodiment of the present invention.
Best Mode for Carrying out the Invention
[0012] Hereinafter the present invention is described on the basis of an embodiment, however, the scope of the present invention is not limited to the embodiment.
[0013] As a substrate for fabrication of the thin polysilicon film, insulating substrates such as glass , quartz , sapphire and so on, or these substrates on which transparent electrodes such as ITO, SnO₂ and so on are formed can be used.
[0014] However, of these substrates, a glass substrate is preferable because of its lower cost, and in particular, an inexpensive soda glass substrate or this substrate deposited transparent electrodes or metal thereon.
[0015] The most important consideration is that since the fabrication temperature of the thin polysilicon film is not more than 400 °C, the diffusion of alkali metals, such as Na, and alkaline earth metals, such as Mg, which are present in the glass substrate, is prevented.
Examples 1∼2 and Comparative Example
[0016] Using a CVD apparatus shown in Fig.1, a polysilicon thin film was deposited onto a glass substrate 3. In the fabricating procedure, firstly an a-Si:H is deposited onto the glass substrate 3 to 20 Å thick by RF discharge; and secondary the film on substrate 3 is exposed for about 20∼about 30 seconds to an ECR plasma generated by a ECR plasma apparatus 5 constructed with permanent-magnets 5a. By repeating this procedure about 300 times, a polysilicon thin film of about 6,000 Å thick is obtained (Example 1). In the drawing, number 1 denotes the chamber, number 2 denotes a substrate rotating device, and number 4 denotes the RF electrodes.
[0017] The condition which the a-Si:H film was formed, were the substrate temperature of 350 °C, the pressure of 0.5 Torr, RF power density of 0.4 W/cm², SiH₄ gas flow rate of 40 SCCM, and H₂ gas flow rate of 200 SCCM. Further the ECR conditions used were: a pressure of 0.02 Torr, a microwave electrical power of 400 W, and an H₂ gas flow rate of 200 SCCM. The Raman spectrum of the fabricated film is shown in Fig.2, and the X-ray diffraction results in Fig.3. From the X-ray diffraction of Fig.3, the polysilicon film is seen to have a strong (110) orientation. In addition, a value of mobility of 20 cm²/Vs was measured for this film using the Van der Paun method. Similary, in the case where the substrate temperature was 250 °C, a polysilicon film was also obtained using the above-described method (Example 2).
[0018] A charactristic feature of the use of the permanent magnet 5a is that under the influence of the magnetic field, the ions are not accelerated towards the substrate 3 as occurs in the case of a standard ECR apparatus, but instead are accelerated towards the center of the diameter of the field. As a result, both the amount and the energy of ions influxing onto the substrate 3 are small compared with general ECR sources using electromagnets. On the other hand, since radicals diffuse in random directions, a similar amount of radicals is produced as with general ECR sources.
[0019] In order to observe the effect of varying the H₂ plasma power, namely, the effect varying the density of Hydrogen radicals, a film was made setting microwave power of 100 W(Comparative Example). In the comparative example, however, no crystallization is observed.
Industrial Applicability
1. A polysilicon thin film formed on a glass substrate characterized in that a hydrogen
content is not more than 5 atomic%.
2. A polysilicon thin film according to Claim 1, wherein a metal electrode or transparent
electrode is formed onto the glass substrate, and the polysilicon thin film is formed
onto said metal or transparent electrode.
3. A polysilicon thin film according to Claim 1 or Claim 2, wherein the polysilicon thin
film is fabricated by repetitions comprising the steps of: firstly fabrication of
an amorphous silicon film by the CVD method or PVD method; and then exposure of said
amorphous silicon film to a hydrogen plasma for a set period of time.
4. A polysilicon thin film according to Claim 3, wherein the hydrogen plasma is generated
by ECR discharge using permanent magnets, and pressure of the plasma is not more than
100 mTorr.
5. A polysilicon thin film according to Claim 1, wherein the polysilicon thin film is
oriented to (110), and strength ratio of (220)/(111) measured by X-ray diffraction
method is not less than 10.
6. A polysilicon thin film according to Claim 1, wherein the polysilicon thin film is
fabricated at a temperature of not more than 400 °C.
7. A method of fabricating thin polysilicon film forming onto a glass substrate characterized
in that the polysilicon thin film is deposited at a temperature of not more than 400
°C.
8. A method according to Claim 7, wherein said manufacturing method comprises multiple
repetitions of a process comprising the steps of: firstly deposition of an amorphous
silicon film by the CVD method or PVD method; and then exposure of said amorphous
silicon film to a hydrogen plasma for a set period of time.
9. A method according to Claim 8, wherein the CVD method or PVD method and the exposure
to a hydrogen plasma are carried out in the same chamber.